Realistically, you can't go wrong with any of them, but there are very distinct differences in them that are reflected in their respective prices.
The 305 is the entry level machine. It has manual adjustable GB, adjustable Threshold, has 3 Noise Cancel channels, is limited to running 7.5kHz or 18.75kHz coils, has 12 notch DISC, choice of 1,2,3, or 12 tone audio, two customizable savable DISC pattern programs, and true All Metal.
The 505 also has Manual GB, adjustable Threshold, has 5 Noise Cancel channels ,can run 3kHz, 7.5kHz, or 18.75kHz coils, has 19 notch DISC, choice of 1,2,3,4, or 19 tone audio, 3 savable DISC programs, and All Metal.
The 705 is the top of the X-terra line, and IMO is the most bang for the buck in the whole detecting world (the best kept secret). It has Manual and Auto GB, GB Tracking, GB Tracking Offset, all of which function in C/T, Beach, and Prospector modes. Like the 505 it has 5 NC channels, can run all three frequency coils, and All Metal, but it also has choice of 1,2,3,4,or 28 tone audio, 28 notch DISC, two Pinpointing modes (ID & sizing), 4 storable DISC patterns, and a backlight!
Considering that I paid $300 for my 705 several years ago, I'd look for a good used one if I were you. There are some really good deals out there if you take the time to look and aren't in a hurry. In general the line is more trouble free than most by any manufacturer, and if you can score one that still has warranty on it you can transfer it.